The following description generally relates to a semiconductor laser diode and a method for fabricating the same, and more particularly to a semiconductor laser diode formed with window at cleavage facet and fabricating method thereof.
A semiconductor laser diode generates a laser beam having frequency of a narrow width and keen directivity and is thus mainly used in a field such as optical communication, a multiple communication and a space communication. Also, the semiconductor laser diode is extensively used for transmission of data or recording and reading of data in a communication field such as an optical communication and an apparatus such as a compact disc player (CDP) and a digital versatile disc player (DVDP). The semiconductor laser diode also may be applicable as devices with improved speed for data transmission or certain read/write functions in players using optical discs.
Among semiconductor light emitting devices, semiconductor laser devices are used as light sources for reading and writing of a signal of an optical recording medium such as CD (Compact Disk), DVD (Digital Versatile Disk) or Blue-Ray Disk. When a semiconductor laser device is used as a light source for writing, a higher-power semiconductor laser device is required because of increased speed and increased capacity of multi-layering media. Therefore, nitride semiconductor laser devices adapted to high power, for each wavelength of infrared, red, blue or the like, have been developed and are stilt now under development.
The extensive use of the semiconductor laser diode is due to facts that the emission characteristics of a laser beam can be maintained in a limited space and the semiconductor laser diode is a compact device and has a small critical current value for emission. An increase in the number of industrial fields adopting the semiconductor laser diode results in an increase in a need for a semiconductor laser diode having a more reduced critical current value. That is, it is required to manufacture an excellent semiconductor laser diode capable of enabling low-current emission and having longer lifetime.
Since a semiconductor laser device may maintain oscillation characteristics of laser beam in a limited space, may be formed to a small scale, and requires a small critical current for laser oscillations, the semiconductor laser device is widely used. As the number of industrial fields to which the semiconductor laser is applied increases, demand for semiconductor laser devices requiring for a smaller critical current increases. In other words, semiconductor laser devices having excellent characteristics, such as oscillating at a low current, and the ability to pass a lifespan test are needed.
For example, a nitride semiconductor laser diode, for example, gallium nitride (GaN)-based laser diode may allow using wavelengths from the ultraviolet region to the visible-green region of the electromagnetic spectrum. The GaN-based laser diodes may be applicable in various areas, including storage/reproduction devices using increased density optical information, laser printers with increased resolution, and projection televisions.
The nitride semiconductor laser diode has gained a good reception in terms of environment-friendly aspect of view due to the fact that it does not use arsenic (As) for main components.
The semiconductor laser diode employed for light source of optical devices must satisfy a single mode and high output power characteristics. To this end, the semiconductor laser diode for light source must be disposed with a ridge waveguide structure that provides a function to confine the current and requires a small critical current for laser oscillations.
The GaN-based laser diodes may have a multi-layered structure of an epitaxial layer including an active layer for emitting laser beam to an upper surface of a sapphire substrate, the epitaxial layer being formed thereon with a ridge, the ridge being formed thereon with a p-electrode, and the epitaxial layer is partially mesa-etched and exposed to form an n-electrode.
When a current is applied from the p-electrode via the ridge, light is produced from the active layer in response to hole-electron recombination. The light produced by the active layer reciprocates between a front cleavage facet and a back cleavage facet, is amplified and emitted outside if resonance conditions are met. A high reflection film is formed on the back cleavage facet to reflect the laser beam to allow the laser beam to be emitted only through the front cleavage facet of the semiconductor laser diode, and the front cleavage facet is formed with an anti-reflection film to prevent the laser beam from being reflected.
Meanwhile, degradation and local segregation may often become severe in the facets of high output semiconductor laser diodes. Poor reliability that shortens the service life of the semiconductor laser diode resulting from degradation of the semiconductor laser diodes on the facet at the light emitting side is known as a big problem in achieving higher power of the semiconductor laser device. This is commonly known as COD (Catastrophic Optical Damage) which is a phenomenon in which the light emitting portion is thermally melted thereby causing emission stop. The reason the COD occurs is that the light emitting portion becomes an absorption region in which laser light is absorbed to generate heat and result in a reduced energy band gap. It is said that non-radiative recombination level is attributable to the absorption region. The light emitting portion of the nitride semiconductor laser device in which laser light emission stops is found to have a hole which may be created as the light emitting portion is melted, and it is understood that the degradation of the light emitting portion causes emission stop.
The conventional nitride semiconductor laser diode is configured in such a manner that p-contact layer and p-pad electrode are formed up to the cleavage facets of the semiconductor laser diode.
If a voltage is applied to the p-pad electrode, holes are introduced along the cleavage facets of the semiconductor laser diode through the p-contact layer, and when the holes are recombined with the electrons to generate light, light absorption becomes high at the cleavage facets to aggravate the degradation on the facets.